Enhancing Vision-Based Structural Displacement Measurement of Civil Structures Through Optical Multiplexing

Vision-based approaches for structural displacement measurement offer the potential to acquire both static deformations and dynamic responses suitable for system identification, while overcoming challenges and limitations associated with conventional displacement sensing techniques. However, many structures of interest, such as bridges, high-rise buildings, and communication, transmission, and wind turbine towers, have geometric dimensions that are considerably larger in a single direction. Consequently, establishing a field of view that encompasses the entire expanse of a structure results in significantly reduced spatial resolution of either natural features of the structure or tracking targets and imposes practical limitations on the ability to resolve motion from the images. These limitations can be addressed by reducing the camera field of view to a small region of the structure, but multiple synchronized cameras would then be required if displacement measurements are sought from many regions across the structure. Optical multiplexing provides an alternative low-cost solution for increasing the effective spatial resolution and facilitating wide field of view by superimposing images from several fields of view into a composite image. This chapter presents the design of a low-cost liquid crystal optical multiplexer assembly with 3D-printed fixture to permit adjustment of the subfields of view. The technique for reconstructing individual subimages from the composite multiplexed images is reviewed and demonstrated using images acquired from a laboratory structure. Independent measurements of structural displacement acquired from displacement transducers and a laser tracker are used to verify the vision-based measurements and characterize the performance of the measurement approach.